If you’ve heard the term ‘blue moon,’ you might picture the Moon painted… blue. But actually, it has nothing to do with color. It’s all about how we track time and lunar cycles.
There are two types of blue moons. Astronomers say the full moon on May 31, 2026, will be one of them—a monthly blue moon.
The Two Definitions of a Blue Moon
The first definition is the seasonal blue moon. This happens when there are four full moons in a season instead of three. The next one will be on August 21, 2032.
The second definition is the monthly blue moon. This occurs when there are two full moons in one calendar month. We will see this on May 31, 2026.
Why Does a Blue Moon Happen?
The Moon takes about 29.5 days to go through all its phases. So, in a year, we usually get 12 full moons. But 12 lunar cycles only add up to about 354 days, which is 11 days short of a calendar year.
This ‘gap’ leads to an extra 13th full moon appearing every two to three years. This extra full moon is called a blue moon.
Even though we say ‘once in a blue moon’ for something rare, this astronomical event isn’t as uncommon as we think.
Why February Never Has a Blue Moon
February has only 28 days (or 29 in leap years), so it can never have two full moons in one month. Sometimes, it might not have any full moons at all, which is called a ‘black moon.’
When the Moon Actually Turns… Blue
There’s also a literal blue moon, but it’s super rare. In 1883, after the huge eruption of Krakatoa volcano in Indonesia, tons of ash shot into the sky. These tiny particles acted like a light filter, scattering red light and giving the Moon a bluish-green hue.
Similar sights have been seen after other big volcanic eruptions like:
- Mount St. Helens (1980),
- El Chichón in Mexico (1983),
- and Pinatubo in the Philippines (1991).
